Low power CD-ROM player for portable computers

ABSTRACT

A computer subsystem of a computer includes a CPU, RAM, display, storage device, input device(s), and a digital-audio generating IC. A CD-ROM subsystem of the computer includes a CD-ROM drive and CD-ROM control buttons for controlling CD-ROM drivers operation while playing audio CDs. An audio-interface IC of the CD-ROM subsystem couples a bus of the computer subsystem to the CD-ROM drive, and to the control buttons. The audio-interface IC, in one operating mode, relays commands and data between the bus and the CD-ROM drive. A second operating mode permits turning the computer subsystem off while the audio-interface IC autonomously responds to the control buttons and transmits commands to the CD-ROM drive for playing an audio CD.

CLAIM OF PROVISIONAL APPLICATION RIGHTS

[0001] This application is a continuation application under 37 C.F.R §1.53(b) of application Ser. No. 09/136,207, filed Aug. 19, 1998, andclaims the benefit of U.S. Provisional Patent Application No. 60/079,508filed on Mar. 26, 1998.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates generally to digital computers and,more particularly, to a digital computer adapted for low power operationwhile playing a CD-ROM.

[0004] 2. Description of the Prior Art

[0005] Portable computers (i.e., notebook, laptop, palmtop and the like)from major original equipment manufacturers such as Toshiba, Compaq,Dell, IBM and others offer CD-ROM drives as either standard or optionaldevices. Notebook, laptop, palmtop computers are aimed at the mobilecomputer user who needs or wants to take work home from the office or ona business trip. An added benefit of CD-ROM equipped portable computersis the opportunity to enjoy periods of relaxation and pleasure byplaying audio tracks from standard music CDs. In the ensuing discussion,the term notebook computer will be understood to apply also to laptop,palmtop and other portable, battery powered computers.

[0006] The Windows operating system's media player or third party audioapplication can play back standard audio CDs on a portable computer.However the simple function of playing an integral audio CD-ROM requiresthat the entire notebook system be powered for the duration of the audioplay back. This causes excessive drain on the notebook's battery powersystem, unnecessarily consuming battery energy better saved for CPUintensive use such as word processing and spreadsheet analysis.

[0007] Conventional laptop and notebook computers typically have severalpower down modes. They can be powered down such that the CPU is almostcompletely off, with the state of the CPU saved on a hard drive. A verylow power portion of the CPU or an auxiliary circuit (e.g. keyboardcontroller) is typically used to recognize when a key is pressed. Thesystem then reactivates normal power to allow the CPU to retrieve thestored machine state from the hard drive thereby restoring the computerinto an operating mode. Some well known power saving modes are calledsleep mode, suspend mode and the like.

[0008] Consequently, a modern energy efficient computer will, over time,operate in several different power management regimes. For example, if aportable computer is being used in an office environment whereelectrical power consumption is an insignificant concern, then thecomputer user may want the computer to provide the highest performanceand availability possible. Conversely, if the computer is being operatedon battery power where there is no convenient source of electricalenergy, then the computer user may want to choose a power managementregime for the computer that will maximize the time the computeroperates without recharging its batteries, even though performance andavailability may be noticeably reduced.

[0009] To facilitate controlling electrical power consumption inpersonal computers, Intel Corporation, Microsoft Corporation, andToshiba Corporation have jointly established an Advanced Configurationand Power Interface Specification (“ACPI Specification”). The ACPISpecification Revision 1.0 of Dec. 22, 1996, Copyright 1996 IntelCorporation, Microsoft Corporation, Toshiba Corporation, establishesboth a set of five (5) Global System States G3—Mechanical Off,G2/S5—Soft Off, G1—Sleeping, G0—Working, S4—Non-Volatile Sleep, and aset of four (4) Device Power States D0—Fully On, through D3—Off. TheACPI Specification defines the Global System States as follows.

[0010] G3 Electrical power is mechanically turned off.

[0011] G2/S5 Electrical power is turned on but the computer consumes aminimal amount of power by not executing either user or system computerprograms, and the system's context is not preserved by hardware.

[0012] G1 Electrical power is turned on, the system's context ispreserved by hardware or system software, but user computer programs arenot being executed.

[0013] G0 Electrical power is turned on and user computer programs areexecuted. In the G0 state, devices such as hard disk drives, CD-ROMdrives, floppy diskette drives, etc are dynamically turned on and off asneeded.

[0014] S4 Electrical power may either be turned off, i.e. Global StateG3, or turned on with the computer consuming a minimal amount of power,i.e. Global State G2/S5, while system context is preserved in anon-volatile storage file before entering either the G3 or G2/S5 state,thereby permitting the computer to be restored to its prior operatingstate, i.e. G1 or G0.

[0015] The ACPI Specification further defines Device Power States asfollows.

[0016] D0 The device is completely active and responsive, and consumesthe most electrical power.

[0017] D1 A lower power state that is defined for different types ofdevices which preserves more device context than the yet lower powerstate D2.

[0018] D2 An even lower power state than D1 that is again defined f ordifferent types of devices, and which preserves less device context thanstate D1.

[0019] D3 Electrical power is fully removed from the device, devicecontext is lost, and system software must reinitialize the device whenit is turned on again.

[0020] The different computer operating modes and associated powermanagement regimes described above are each characterized by a uniquepower demand (i.e., current drain) from the battery power supply. Thisis an important feature both in design of portable computer systems, andin marketing them as well. Great attention is focused on minimizing thepower demand for each of the different Global and Device operatingmodes. Thus, the power demand characterizing each power managementregime is a critical factor to be considered for portable computers,particularly one that includes a CD-ROM drive for playing audio CDs.

[0021] In implementing conventional computer power managementstrategies, a power management routine (“PMR”) executed by the CPU mustperiodically monitor peripheral devices to assess whether a peripheraldevice's operation may be suspended. Similarly, if it becomes necessaryto access a peripheral device whose operation has been suspended such asin Device Power modes D1-D3, the PMR must resume that peripheraldevice's operation. Generally, suspending the operation of a peripheraldevice and resuming its operation respectively require that the PMRexecuted by the CPU perform a unique sequence of operations in turningoff electrical power to a peripheral device, and in turning electricalpower back on. Writing a computer program that detects a need to executea power-on or a power-off sequence of operations for a peripheral deviceis a cumbersome task.

[0022] Previous portable computers that include a CD-ROM use PMRfunctions to minimize battery drain. However, if CPU operation has beensuspended to save electrical power, the computer can essentially donothing. Therefore, in the minimal power drain mode, the CPU cannot usethe windows operating system's media player or third party audioapplication to play audio CDs.

[0023] A significant power drain in portable computers occurs in theconventional LCD monitor. Typically, 60 to 70% of the power consumed bya notebook is consumed by the display. Thus even if a computer'sdevices, including even perhaps the CPU, were in a lower power state,i.e., one of the lower Device Power States D1-D3 for power savingsduring CD-ROM play only, the need to use the normal LCD to displayCD-ROM status and the music playing status would itself impedesignificantly reducing power consumption.

[0024] For the reasons described above, it is apparent that adisadvantage of present portable computers for playing audio CDs is thatsome portion of the computer system must remain energized state todetect key actuation and then to restore power or activate a powerrestore function of the CPU and associated peripherals (e.g. hard drive,keyboard controller, display, etc.). At times when a portable computeris being used during travel, or when line power is otherwiseunavailable, the user may wish to play some audio CDs. Given the limitedbattery life of most portables, e.g., 3 to 5 hours of use, the user mayhave to choose to forego using the CD-ROM capability for very long, outof fear that the notebook will not be functional for needed work orcommunication.

BRIEF SUMMARY OF THE INVENTION

[0025] The present invention extends the playing time for a CD-ROMequipped notebook computer while minimizing the loss of potentialoperating time as a computer.

[0026] Another advantage is to select and control the music being playedwithout engaging (powering on) the CPU or other notebook peripherals,i.e., hard drive, display, memory and the like.

[0027] Another advantage is to extend playing time and lower power drainwhile minimizing the requirements for additional software drivers.

[0028] Another advantage would be to provide CD-ROM/music status to theuser without using the normal display screen with its typically highbattery drain requirements.

[0029] Another advantage of the present invention is that it facilitatesportable computer product differentiation by providing various differentaudio CD playing user interfaces.

[0030] Another advantage of the present invention is that it permitsdesigners of portable computer systems to choose among various differentaudio CD playing user interfaces.

[0031] Another advantage of the present invention is that all of thepreceding advantages may be obtained merely by inserting an IC inaccordance with the present invention into an existing portable computerdesign.

[0032] In one embodiment the present invention is a digital computerthat includes both a computer subsystem and a CD-ROM subsystem. Thecomputer subsystem is conventional and includes a digital computer busvia which various digital computer devices included in the computersubsystem exchange commands and data. Devices included in the computersubsystem include a central processing unit (“CPU”), a random accessmemory (“RAM”), a display, a read-write mass storage device, a manualinput device, and a digital-audio generating integrated circuit (“IC”).

[0033] The CD-ROM subsystem includes a conventional CD-ROM drive and anaudio output amplifier that is coupled to the CD-ROM drive for receivingan analog audio signal from the CD-ROM drive. The CD-ROM subsystem alsoincludes several CD-ROM control buttons for controlling operation of theCD-ROM drive during replay of audio compact disks (“CDs”). Anaudio-interface IC, also included in the CD-ROM subsystem, is coupled tothe digital computer bus of the computer subsystem, to the CD-ROM drive,and to the CD-ROM control buttons. The audio-interface IC, in a firstoperating mode in which the computer subsystem is energized andoperating, relays commands and data between the digital computer bus ofthe computer subsystem and the CD-ROM drive. In a second operating modein which the computer subsystem is not energized and is inoperative, theaudio-interface IC autonomously responds to signals received from theCD-ROM control buttons and transmits commands to the CD-ROM drive whichcause the CD-ROM drive to play an audio CD present in the CD-ROM drive.

[0034] In a particularly preferred embodiment the present invention theaudio-interface IC has a third operating mode in addition to the twodescribed above. In this third operating mode the computer subsystem isenergized and operating, and the audio-interface IC receives commandsfrom the CD-ROM control buttons and stores such commands for subsequentretrieval by a computer program executed by the CPU included in saidcomputer subsystem. Furthermore, in this third operating mode, asdirected by a computer program executed by the CPU, the audio-interfaceIC either merely relays commands and data between the computersubsystem's digital computer bus and the CD-ROM drive, or independentlyresponds to CD-ROM button commands by generating CD-ROM commandsinternally, and independently transmitting such commands to the CD-ROMdrive to control playing an audio CD present in the CD-ROM drive.

[0035] An advantage of the present invention is that during playing ofaudio CDs the invention can double the interval during which the digitalcomputer can operate on a particular amount of battery power.

[0036] These and other features, objects and advantages will beunderstood or apparent to those of ordinary skill in the art from thefollowing detailed description of the preferred embodiment asillustrated in the various drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0037]FIG. 1 is a block diagram depicting a digital computer inaccordance with the present invention having both a computer subsystemand a CD-ROM subsystem that includes an audio-interface IC;

[0038]FIG. 2 is a block diagram depicting an audio-interface IC inaccordance with the present invention;

[0039]FIG. 3 is a register diagram depicting contents of a registerblock included in the audio-interface IC illustrated in FIG. 2;

[0040]FIG. 4 is a state diagram depicting operation of theaudio-interface IC illustrated in FIG. 2 if the computer subsystem isnot energized and is inoperative;

[0041]FIG. 5 is a flow diagram depicting operation of the computertogether with the audio-interface IC illustrated in FIG. 2 when thecomputer subsystem is energized and the audio-interface IC operates inthe third operating mode; and

[0042]FIG. 6 is a block diagram depicting a circuit preferably includedin the audio-interface IC for interfacing between a bus included in thecomputer subsystem of the digital computer and a CD-ROM drive.

DETAILED DESCRIPTION

[0043]FIG. 1, depicts a system block diagram of an exemplarybattery-powered portable computer 100 adapted for reading digitalcomputer data from a compact disk-read only memory (“CD-ROM”), and forplaying audio CDs. For purposes of playing audio CDs and powermanagement, the computer 100 is functionally partitioned into a computersubsystem 104 and a CD-ROM subsystem 106.

The Computer Subsystem 104

[0044] The computer subsystem 104 includes all conventionaldata-processing components such as a microprocessor-based centralprocessing unit and random access memory (“RAM”) system (“CPU system”)120 together with various ICs described in greater detail below. Thecomputer 100 also conventionally includes a display 110 (TFT LCD matrixdisplay, CRT and the like), manual input devices 112 (e.g. keyboard,mouse, touch-pad), and read-write mass storage device 114 (e.g. harddrives, floppy drives, optical drives and the like), which operateconventionally within the computer 100.

[0045] The computer subsystem 104 includes associated input/output(“I/O”) buses (e.g. PCI bus 116, and ISA bus 118) for interconnectingvarious subsystems included in the computer 100. In the computer 100,the CPU 120 (e.g. a Pentium microprocessor) exchanges data with the PCIbus 116 through a system controller IC 122 (e.g. Intel 82439HX known asthe “Northbridge”) for controlling on-board L2 cache. The systemcontroller IC 122 is described in a document, a copy of which may beretrieved from http://developer.intel.com/design/intarch/TOP_(—)800.HTM,that is hereby incorporated by reference. Via suitable adapter devices,conventionally the PCI bus 116 permits the CPU 120 to exchange data withhigher performance devices such as the display 110 and the read-writemass storage device 114.

[0046] A bus bridge IC 124 (e.g. a 82371SB “Southbridge” IC)interconnects the PCI bus 116 with the ISA bus 118 and with an IDE bus128. A description of the bus bridge IC 124 can be obtained fromhttp://developer.intel.com/design/intarch/embdmodl.htm and is herebyincorporated by reference. The bus bridge IC 124 is available from IntelCorp., Santa Clara, Calif. Via suitable adapter devices, conventionallythe ISA bus 118 permits the CPU 120 to exchange data with lower speeddevices such as the manual input devices 112. However, for particularconfigurations of the computer 100, the CPU 120 may exchange data withhigher performance devices, such as the read-write mass storage device114, via the ISA bus 118, or directly via the IDE bus 128.

[0047] A digital-audio generation IC 130 included in the computersubsystem 104 communicates with the CPU 120 either via the ISA bus 118,or via the PCI bus 116 as indicated by a pair of dashed lines 126. Thedigital-audio generation IC 130 is conventional and may be either aMaestro-1™ or Maestro-2™ marketed by ESS Technology, Inc. of Fremont,Calif.

[0048] As is well known to those skilled in the art, the computerprograms executed by the microprocessor included in the CPU 120 of alaptop or notebook computer 100 usually include Power ManagementRoutines (PMRs). Under appropriate operating conditions, the PMRs mayplace the computer 100 into one of the several power managementoperating modes such as those described previously for the ACPISpecification. Computer programs that place the computer 100 into one ofthe various power management operating modes may be prepared by onehaving ordinary skill in the art, and form no part of the presentinvention. Each of the various power management operating modes ischaracterized by a corresponding battery power requirement. For example,the current demand for the ACPI Specification's Device Power States maybe those set forth below. Device Power Battery Power State Required D0 -Fully On Ir1 D1 Ir2 < Ir1 D2 Ir3 < Ir2 D3 - Off Ir4 < Ir3

The CD-ROM Subsystem 106

[0049] The CD-ROM subsystem 106 includes an audio-interface IC 102 inaccordance with the present invention, a CD-ROM drive 138, CD-ROMcontrol buttons 142, an icon liquid crystal display (“LCD”) 144, atrack-number display 147, an audio output amplifier 146, and audiooutput transducer 148, e.g. speakers or headphones. The CD-ROM controlbuttons 142, which connect to audio-interface IC 102 via acontrol-button bus 143, include buttons for playing or pausing an audioCD, for fast-forwarding the audio CD, for rewinding the audio CD, andfor stopping or ejecting the audio DC. Thus, the CD-ROM control buttons142 permit a user of the computer 100 to completely control operation ofthe CD-ROM drive 138 while playing an audio CD. The CD-ROM drive 138 isa conventional CD-ROM drive capable of operating with conventional ATAPIinterface commands provided through an IDE-bus extension 129 whichoriginates at the audio-interface IC 102. The CD-ROM subsystem 106 alsoincludes an audio signals bus 152 that supplies left and right channelstereo audio signals directly to the audio output amplifier 146.

[0050] Depending upon the operating mode of the computer 100, an audioswitch 154, which operates in response to control signals received fromthe audio-interface IC 102, may couple the left and right channel stereoaudio signals to the digital-audio generation IC 130 included in thecomputer subsystem 104. When the computer subsystem 104 is notenergized, to reduce electrical power consumption caused by leakageelectrical currents in ICs included in the computer subsystem 104:

[0051] 1. the audio switch 154 electrically isolates the audio signalsbus 152 from the CD-ROM drive 138; and

[0052] 2. the audio-interface IC 102 correspondingly electricallyisolates itself from the IDE bus 128.

[0053] A loudness control-signal line 156 couples a volume controlsignal from the audio-interface IC 102 to the audio output amplifier146. As described in greater detail below, the audio-interface IC 102provides a signal to the icon LCD 144 via a LCD-signal line 145 forindicating that the CD-ROM drive 138 is operating. The audio-interfaceIC 102 provides signals to the track-number display 147 via atrack-number-display bus 149 for displaying a track number as an audioCD is played.

[0054] A System Management Bus (“SMBus”) 162 permits the audio-interfaceIC 102 to exchange commands and data with the computer subsystem 104.Within the computer subsystem 104, the SMBus 162 connects to the ISA bus118 via a keyboard controller IC 164 as indicated in FIG. 1, or theSMBus 162 may connect directly to the bus bridge IC 124, as indicated bya dashed line 166 in FIG. 1.

[0055] Both the computer subsystem 104 and the CD-ROM subsystem 106receive electrical power directly from a battery, not illustrated in anyof the FIGS. Depending upon the operating mode of the computer 100 forplaying audio CDs, either the CD-ROM subsystem 106 alone, or both thecomputer subsystem 104 and the CD-ROM subsystem 106 may be energized. Ifthe computer subsystem 104 receives no electrical power, then operationof the CD-ROM drive 138 is effected completely within the CD-ROMsubsystem 106 with the audio-interface IC 102 originating signals forcontrolling operation of the CD-ROM drive 138. If the computer subsystem104 is energized and operating, then operation of the CD-ROM drive 138can be effected, via the audio-interface IC 102, by commands receivedfrom a computer program executed by the 120.

Interconnections Between the Computer Subsystem 104 and the CD-ROMSubsystem 106

[0056] The audio-interface IC 102 exchanges commands and data with acomputer program executed by the CPU 120 through the IDE bus 128 and theSMBus 162. The computer subsystem 104 also selectively supplies left andright audio signals from the CD-ROM drive 138 to the digital-audiogeneration IC 130 depending upon the operating mode of the computer 100.During operation of the computer 100 in which the computer subsystem 104is energized and operating, the audio-interface IC 102 relays commandsand data between the computer subsystem 104 and CD-ROM drive 138.Commands and data which the computer subsystem 104 exchanges with theCD-ROM drive 138 are well known to those skilled in the art.

General Description of the Audio-Interface IC 102

[0057] Referring now to a detailed block diagram of the audio-interfaceIC 102 depicted in FIG. 2, the audio-interface IC 102 includes a statemachine 202 which controls overall operation of the audio-interface IC102. Inclusion of the state machine 202 in the audio-interface IC 102,as contrasted with a programmable controller, provides betterperformance. The state machine 202 connects via an internal bus 204 to aSMBus interface 206, to a register block 208, to a LCD control 212, andto a clock generator 214.

[0058] The SMBus interface 206, which is coupled to the SMBus 162,issues an interrupt signal from an INTN pin 222 whenever a user of thecomputer 100 presses any of the CD-ROM control buttons 142. The INTN pin222 may be advantageously connected to a SMBus alert signal-line of theSMBus 162. Upon receiving a SMBus alert signal, a computer programexecuted by the CPU 120 can interrogate the register block 208 includedin the audio-interface IC 102 via the SMBus 162 to determine which ofthe CD-ROM control buttons 142 has been pressed.

[0059] In addition to being coupled to the internal bus 204, theregister block 208 is also coupled to an operating-mode bus 226 by whichthe computer 100 supplies operating mode signals to the audio-interfaceIC 102 via the operating-mode bus 226 include a reset signal which uponactivation causes the audio-interface IC 102 to be reset. Theoperating-mode bus 226 also supplies a signal to the audio-interface IC102 that indicates whether the computer subsystem 104 is energized. And,the operating-mode bus 226 supplies a signal to the audio-interface IC102 which indicates if the device connected to the IDE-bus extension 129is a CD-ROM drive. Supplying a signal to the audio-interface IC 102which indicates whether a CD-ROM drive is connected to the IDE-busextension 129 avoids a requirement that the audio-interface IC 102 firstpower on and then interrogate a device connected to the IDE-busextension 129 to determine whether such device is a CD-ROM drive or ahard disk drive. Avoiding this interrogation requirement is particularlyadvantageous if the computer 100 permits a user to readily interchange aCD-ROM drive for some other device such as a hard disk drive orconversely.

[0060] As is readily apparent, the CD-ROM control buttons 142, icon LCD144 and track-number display 147 of the CD-ROM subsystem 106 provide auser of the computer 100 with a self-contained interface for playingaudio CDs. To effect this functionality, the audio-interface IC 102includes control-button logic 232 that receives electrical signals fromthe CD-ROM control buttons 142 via the control-button bus 143. Inresponse to such signals, the control-button logic 232 may store datainto the register block 208, or it may cause a digital volume control236 to transmit control signals via the loudness control-signal line 156to the audio output amplifier 146. Similarly, when the computersubsystem 104 is playing an audio CD the LCD control 212 transmits asignal via the LCD-signal line 145 to activate an audio playback iconincluded in the icon LCD 144. And during such audio CD playback the LCDcontrol 212 transmits signals via the track-number-display bus 149 whichcause the track-number display 147 to display a number that indicatesthe current track of an audio CD. As described in greater detail below”data indicating the current track number is present in the registerblock 208.

[0061] When the computer subsystem 104 is energized, a host IDEinterface 242 included in the audio-interface IC 102 couples electricalsignals between the IDE bus 128 and the IDE-bus extension 129 via anIDE-signals multiplexer 244 included in the audio-interface IC 102. Theelectrical signals coupled between the IDE bus 128 and the IDE-busextension 129 effect exchanges of commands and data between a computerprogram executed by the CPU 120 and the CD-ROM drive 138. However, ifonly the CD-ROM subsystem 106 is energized, then the state machine 202exchanges electrical signals with the IDE-bus extension 129 via theIDE-signals multiplexer 244. And as set forth above, if the computersubsystem 104 is not energized the host IDE interface 242 electricallyisolates the audio-interface IC 102 from the IDE bus 128 of the computersubsystem 104 to reduce, as much as possible, leakage current flowingfrom the CD-ROM subsystem 106 into the computer subsystem 104. Set forthbelow is a table listing commands in accordance with the ATAPI protocolwhich the state machine 202 may transmit to the CD-ROM drive 138 via theIDE-signals multiplexer 244 and IDE-bus extension 129. Command OpcodePause/Resume 4Bh Play Audio MSF 47h Stop 1Bh Read Table of Contents 43hRequest Sense 03h Read Sub Channel 42h Test Unit Ready 00h Lock/Unlock1Eh Inquiry 12h Sleep E6h

[0062] The state machine 202 also transmits a pair of signals from theaudio-interface IC 102 via a mode-control signal bus 246. A mode signalsupplied by the state machine 202 to the mode-control signal bus 246indicates that the audio-interface IC 102 is operating for receivingsignals from the CD-ROM control buttons 142. An electrical power controlsignal supplied by the state machine 202 to the mode-control signal bus246 indicates either that the CD-ROM drive 138 is or has been recentlyoperating, or that the CD-ROM drive 138 has not been operating recently.This signal may be used within the CD-ROM subsystem 106 forappropriately controlling the supply of electrical power to the CD-ROMdrive 138 and/or the audio output amplifier 146.

[0063] During power-on initialization of the computer 100, input signalssupplied to the audio-interface IC 102 via the mode-control signal bus246 respectively select an address for the audio-interface IC 102 on theSMBus 162, and also control whether the state machine 202 transmits thesignal for controlling the supply of electrical power to the CD-ROMdrive 138 and/or the audio output amplifier 146.

[0064] The audio-interface IC 102 also receives a pair of signals via amode-set signal bus 248 that specify particular hardware characteristicsof the CD-ROM subsystem 106. One of these signals specifies which one oftwo (2) different types of volume control ICs is included in the audiooutput amplifier 146. The other mode-set signal specifies a particularcharacteristic for signals transmitted from the audio-interface IC 102to the track-number display 147 via the track-number-display bus 149.

[0065] The clock generator 214 included in the audio-interface IC 102connects to an oscillator-in signal-line 252 and to an oscillator-oursignal-line 254. The oscillator-in and -out signal-lines 252 and 254connect to an 8 MHz crystal external to the audio-interface IC 102 thatis not separately depicted in any of the FIGS.

Register Block 208

[0066]FIG. 3 illustrates registers 208 a-208 h included in the registerblock 208. Seven high order bits of a low-order byte 208 aa of a Chipand Revision ID Register 208 a stores a programmable address for theaudio-interface IC 102 on the SMBus 162. A high-order byte 208 ab of theChip and Revision ID Register 208 a stores a revision number for theaudio-interface IC 102.

[0067] Respective states stored in software programmable bits 0, 1, 3,and 5 in a low-order byte of a Control-Buttons Change-Register 208 bstore data indicating pressing of specific CD-ROM control buttons 142.Bit 208 ba 0 in the stores data indicating that a rewind button has beenpressed. Bit 208 ba 1 stores data indicating that a fast-forward buttonhas been pressed. Bit 208 ba 3 in the Control-Buttons Change-Register208 b stores data indicating that a stop/eject button has been pressed.And bit 208 ba 5 in the Control-Buttons Change-Register 208 b storesdata indicating that a play/pause button has been pressed. Softwareprogrammable bit 208 hb 0 of a high-order byte of an Interrupt StatusRegister 208 h stores data indicating that one of the CD-ROM controlbuttons 142 has been pressed, and causes the interrupt signal to betransmitted from the audio-interface IC 102 via the INTN pin 222.Because bits oil 1, 3 and 5 in the Control-Buttons Change-Register 208 bare software programmable, a computer program executed by the CPU 120can assign data values, i.e. 0 or 1, to any of them.

[0068] A state of bit 208 ba 7 of the Control-Buttons Change-Register208 b indicates whether the audio-interface IC 102 is enabled fortransmitting the ATAPI protocol commands listed above to the CD-ROMdrive 138. A state of bit 208 ca 1 in a low-order byte of an ElectricalPower Register 208 c enables the audio-interface IC 102 for controllingelectrical power to the CD-ROM drive 138 and to the audio outputamplifier 146. If bit 208 ca 1 is enabled, a state of bit 208 ca 0 inthe Electrical Power Register 208 c controls the supply of electricalpower to the CD-ROM drive 138 and audio output amplifier 146.

[0069] Respective states stored in software programmable bits 0, 1 and 2in a low-order byte of a Command Control Register 208 d store data forcontrolling operation of the audio-interface IC 102 when the computersubsystem 104 is energized. Bit 208 da 0 in the Command Control Register208 d stores data which controls whether the audio-interface IC 102 isenabled for executing a single instance of a function specified, asdescribed above, by data values that are assigned to bits in theControl-Buttons Change-Register 208 b by pressing the CD-ROM controlbuttons 142. Bit 208 da 1 stores data which prevents the audio-interfaceIC 102 from executing any function specified by data values assigned bypressing the CD-ROM control buttons 142 to bits in the Control-ButtonsChange Register 208 b. Bit 208 da 2 stores data which indicates whetherthe audio-interface IC 102 has executed a single instance of a functionspecified by data values assigned by pressing the CD-ROM control buttons142 to bits in the Control-Buttons Change-Register 208 b.

[0070] A bit 208 ea 0 in a low-order byte of an Operating Mode Register208 e stores data which specifies a specific operating mode of theCD-ROM subsystem 106, i.e. the third operating mode, to be described ingreater detail below. When the audio-interface IC 102 operates in thethird operating mode, bit 208 ea 4 in the operating Mode Register 208 estores data which specifies selection of a particular drive, i.e. amaster drive or a slave drive, to receive ATAPI protocol commands fromthe audio-interface IC 102.

[0071] Bit 208 eb 1 in a high-order byte of the Operating Mode Register208 e controls application of a clock signal to the state machine 202.When the audio-interface IC 102 operates in the mode in which it merelyrelays data and ATAPI commands between the IDE bus 128 and the IDE-busextension 129, to conserve energy bit 208 eb 1 may be set therebyhalting application of the clock signal to the state machine 202.

[0072] Bits 0-6 of a low-order byte 208 ga of a Track Number Register208 g store a track number read from an audio CD while it is beingplayed. Software programmable bits 0-6 of a high-order byte 208 gb storea track number displayed on the track-number display 147.

Operating Modes

[0073] If the computer subsystem 104 is energized and operating, in oneoperating mode of the CD-ROM subsystem 106 the audio-interface IC 102transparently relays commands and data between the IDE bus 128 and theCD-ROM drive 138. FIG. 4 depicts states and state transitions of theaudio-interface IC 102 for an operating mode of the computer 100 inwhich the computer subsystem 104 is not energized and is inoperative.When the computer subsystem 104 is not energized and the CD-ROMsubsystem 106 is initially energized, or immediately after theaudio-interface IC 102 is reset by a signal received from theoperating-mode bus 226, the audio-interface IC 102 enters an initializestate 302 depicted in FIG. 4.

[0074] A pressing of the play/pause button when the audio-interface IC102 is in the initialize state 302 causes the audio-interface IC 102 totransition to a play state 304 in which the audio-interface IC 102transmits commands in accordance with the ATAPI protocol to the CD-ROMdrive 138 that cause the CD-ROM subsystem 106 to play an audio CD. Ifthe audio-interface IC 102 is in the play state 304, then pressing thestop or eject button, or reaching the end of all the audio CD trackscauses the audio-interface IC 102 to re-enter the initialize state 302and to return to the beginning of the audio CD.

[0075] While the CD-ROM subsystem 106 is in the play state 304 playingan audio CD, pressing either the fast-forward or rewind buttons causesthe audio-interface IC 102 to enter a fast-forward-or-rewind state 306.In the fast-forward-or-rewind state 306 the audio-interface IC 102transmits commands to the CD-ROM drive 138 that either fast-forward orrewind the audio CD. If the CD-ROM drive 138 completes the fast-forwardor rewind command, or reaches the end or beginning of the audio CDtrack, the audio-interface IC 102 re-enters the initialize state 302.While the CD-ROM subsystem 106 is fast-forwarding or rewinding an audioCD, pressing the play button causes the audio-interface IC 102 to enterthe play state 304 and resume playing the audio CD at the beginning ofthe present track.

[0076] While the CD-ROM subsystem 106 is in the play state 304 playingan audio CD or in the fast-forward-or-rewind state 306 fast-forwardingor rewinding an audio CD, pressing the pause button causes theaudio-interface IC 102 to enter a pause state 308 which pauses operationof the CD-ROM drive 138. If the audio-interface IC 102 is in the pausestate 308, pressing the play button causes the audio-interface IC 102 toenter the play state 304 and resume playing the audio CD at the presentlocation in the track, pressing either the fast-forward or rewindbuttons causes the audio-interface IC 102 to enter the pause state 308,and pressing the stop button causes the audio-interface IC 102 to enterthe initialize state 302.

[0077] If the audio-interface IC 102 is in the initialize state 302, anda signal has been supplied to the audio-interface IC 102 via themode-set signal bus 248 which enables the state machine 202 forcontrolling the supply of electrical power to the CD-ROM drive 138and/or the audio output amplifier 146, and a pre-established two (2)minute interval passes during which none of the CD-ROM control buttons142 are pressed; then the audio-interface IC 102 enters a sleep state312. Upon entering the sleep state 312 the audio-interface IC 102 sendsan ATAPI protocol sleep command to the CD-ROM drive 138 thereby slowingdown a clock included in the CD-ROM drive 138. If the audio-interface IC102 is in the sleep state 312 and a second, pre-established two (2)minute interval passes during which none of the CD-ROM control buttons142 are pressed, then the audio-interface IC 102 enters a suspend state314 in which the audio-interface IC 102 transmits a signal via themode-control signal bus 246 which indicates that the CD-ROM drive 138has not been operating recently. Electrical circuitry included in theCD-ROM subsystem 106 may use this signal from the audio-interface IC 102for removing electrical power from both the CD-ROM drive 138 and fromthe audio output amplifier 146. If the audio-11 interface IC 102 iseither in the sleep state 312 or in the suspend state 314, then pressingany of the CD-ROM control buttons 142 causes the audio-interface IC 102to re-enter the initialize state 302.

[0078] In addition to the two operating modes described above, thepreferred embodiment of the audio-interface IC 102 may be configured tooperate a third operating mode. In this third operating mode thecomputer subsystem 104 is energized and operating, the audio-interfaceIC 102 receives commands from the CD-ROM control buttons 142, and storessuch commands into the Control-Buttons Change-Register 208 b forsubsequent retrieval by a computer program executed by the CPU 120. Theflow diagram of FIG. 5 depicts operation of the computer 100 includingthe audio-interface IC 102 for playing audio CDs in this third operatingmode. Thus as depicted in FIG. 5, in the third operating mode while auser does not press any of the CD-ROM control buttons 142 the statemachine 202 loops at decision block 372 waiting for one of the CD-ROMcontrol buttons 142 to be pressed. If any of the CD-ROM control buttons142 are pressed, the state machine 202 in processing block 374 sets thebit 208 hb 0 in the Interrupt Status Register 208 h thereby causing theSMBus interface 206 to transmit a SMBus interrupt INTN via the INTN pin222 either to the keyboard controller IC 164, or to the bus bridge IC124. In addition to transmitting a SMBus interrupt, the state machine202 in processing block 374 also sets the appropriate bit in theControl-Buttons Change-Register 208 b to indicate which of the CD-ROMcontrol buttons 142 has been pressed. The computer program executed bythe CPU 120 in processing block 376 responds to receipt of the INTNinterrupt by reading the contents of the Control-Buttons Change-Register208 b, and by then transmitting data to the CPU 120 via the SMBus 162that resets the bit 208 hb 0 of the Interrupt Status Register 208 hthereby clearing the INTN interrupt.

[0079] After the computer program executed by the CPU 120 clears theINTN interrupt, the state machine 202 in decision block 378 determineswhether the computer program executed by the CPU 120 has previously setbit 208 da 1 in the Command Control Register 208 d. If bit 208 da 1 isnot set, then the state machine 202 in decision block 382 determineswhether the computer program executed by the CPU 120 has previously setbit 208 da 0 in the Command Control Register 208 d. If bit 208 da 0 isset, then the state machine 202 in processing block 384 transmits to theCD-ROM drive 138 via the IDE-bus extension 129 the ATAPI command(s)which cause the IDE bus 128 to respond appropriately to the pressing ofthe CD-ROM control button 142. If bit 208 da 1 has been set, and bit 208da 0 has not been set, then the state machine 202 merely passes throughprocessing block 386 without sending any commands to the CD-ROM drive138.

[0080] After performing either processing block 384 or processing block386, the state machine 202 in processing block 392 again sets bit 208 hb0 in the Interrupt Status Register 208 h thereby again causing the SMBusinterface 206 to transmit a SMBus interrupt via the INTN pin 222 eitherto the keyboard controller IC 164, or to the bus bridge IC 124. Asbefore, the computer program executed by the CPU 120 in processing block394 responds to receipt of the INTN interrupt by transmitting data tothe CPU 120 via the SMBus 162 that resets bit 208 hb 0 of the InterruptStatus Register 208 h thereby clearing the INTN interrupt. After thecomputer program executed by the CPU 120 resets bit 208 hb 0, the statemachine 202 returns to decision block 372 to resume waiting for one ofthe CD-ROM control buttons 142 to be pressed.

[0081]FIG. 6 depicts a preferred circuit for interfacing between the IDEbus 128, the IDE-bus extension 129 and the audio-interface IC 102. Whilethe host IDE interface 242 and the IDE-signals multiplexer 244 of theaudio-interface IC 102 may employ conventional IC I/O buffers, suchconventional circuits will appreciably delay signals passing through theaudio-interface IC 102 between the IDE bus 128 and the IDE-bus extension129. Rather than employing conventional IC I/O buffer circuits for thehost IDE interface 242 and the IDE-signals multiplexer 244, thepreferred circuit depicted in FIG. 6 employs transmission gates 402interposed between the IDE bus 128 and the IDE-bus extension 129 forselectively either coupling them together, or isolating them from eachother. Use of the transmission gates 402 rather than conventional IC I/Obuffers significantly reduces delay for signals passing through theaudio-interface IC 102 between the IDE bus 128 and the IDE-bus extension129.

[0082] Analogously to the transmission gates 402, signals that the statemachine 202 receives from the IDE-bus extension 129 are also selectivelycoupled to or isolated from input buffers 404 for the state machine 202by a transmission gates 406 also in response to a signal generated bythe state machine 202. Alternatively, signals that the state machine 202transmits to the IDE-bus extension 129 pass through output drivers 408when the output drivers 408 are enabled by a control signal supplied bythe state machine 202. This preferred configuration for exchangingsignals between the state machine 202 and the IDE bus extension 129 alsopermits isolating the state machine 202 from input signals on theIDE-bus extension 129 during certain critical events such as when theCD-ROM drive 138 is removed to be replaced by another device that is tobe coupled to the IDE bus 128 without removing electrical power from thecomputer subsystem 104 and CD-ROM subsystem 106.

[0083] The transmission gates 402, input buffers 404, transmission gates406, and output drivers 408 collectively constitute an analogmultiplexer that, in the illustration of FIG. 6, is enclosed within adashed line 412. Enabling operation of the analog multiplexer 412permits exchanging signals between the IDE-bus extension 129 and eitherthe IDE bus 128 or state machine 202. Disabling operation of the analogmultiplexer 412 isolates the IDE-bus extension 129 both from the IDE bus128 and from the state machine 202. Inclusion of the analog multiplexer412 in the audio-interface IC 102 permits various IDE devices, such asthe CD-ROM drive 138, a hard disk, a Digital Video Disk (“DVD”) drive, aZIP drive, or a Superdisk, to be connected to or disconnected from theIDE-bus extension 129 of a fully operational computer 100.

[0084] Operation of the analog multiplexer 412 is enabled or disabled bya signal coupled from a multiplexer control 414 included in theaudio-interface IC 102. The multiplexer control 414 transmits thecontrol signal to the transmission gates 402 and 406 via atransmission-gate-control signal-line 416. For disabling operation ofthe analog multiplexer 412, the multiplexer control 414 receivessoftware data via the SMBus 162. For enabling operation of the analogmultiplexer 412, the multiplexer control 414 may receive either or botha hardware signal via a touchdown signal-line 418 and/or software datavia the SMBus 162. The multiplexer control 414 also receives signalsfrom the state machine 202 via a multiplexer-control-signalselection-bus 422. Signals supplied to the multiplexer control 414 fromthe state machine 202 via the multiplexer-control-signal selection-bus422 determine whether operation of the multiplexer control 414 isenabled by the signal supplied via the touchdown signal-line 418, or bydata supplied to the multiplexer control 414 via the SMBus 162.

[0085] Accordingly, as illustrated in FIG. 3 the register block 208 ofthe audio-interface IC 102 which employs the preferred analogmultiplexer 412 includes a software programmable bit 208 ca 3 located inthe low-order byte of the Electrical Power Register 208 c. Setting bit208 ca 3 activates software data control of the multiplexer control 414via the SMBus 162, while resetting bit 208 ca 3 activates hardwareenablement of the multiplexer control 414 via the touchdown signal-line418. To effect software disabling and enabling of the multiplexercontrol 414, the low-order byte of the Electrical Power Register 208 calso includes a software programmable bit 208 ca 2 which if set enablesoperation of the transmission gates 402 and 406. Conversely, resettingbit 208 ca 2 disables operation of the transmission gates 402 and 406.

[0086] When the computer 100 is fully operational with a deviceconnected to the IDE-bus extension 129, using one of the manual inputdevices 112 a user of the computer 100 may direct a computer programexecuted by the CPU 120 to release the device connected to the IDE-busextension 129. In response thereto, the computer 100 causes the analogmultiplexer 412 to isolate the IDE-bus extension 129 from the remainderof the CD-ROM subsystem 106, and then mechanically releases the deviceso it may be physically removed from the computer 100. Upon subsequentinsertion of a device into the computer 100 and connection of thatdevice to the IDE-bus extension 129, the device becomes mechanicallylocked into the computer 100, and the audio-interface IC 102 responsive,either the hardware signal or software data, re-couples the IDE-busextension 129 to the remainder of the CD-ROM subsystem 106 therebyrestoring the computer 100 to full operation.

[0087] Although the present invention has been described in terms of thepresently preferred embodiment, it is to be understood that suchdisclosure is purely illustrative and is not to be interpreted aslimiting. Consequently, without departing from the spirit and scope ofthe invention, various alterations, modifications, and/or alternativeapplications of the invention will, no doubt, be suggested to thoseskilled in the art after having read the preceding disclosure.Accordingly, it is intended that the following claims be interpreted asencompassing all alterations, modifications, or alternative applicationsas fall within the true spirit and scope of the invention.

What is claimed is:
 1. A digital computer comprising both a computersubsystem and a CD-ROM subsystem: said computer subsystem including adigital computer bus via which various digital computer devices includedin the computer subsystem exchange commands and data, such devicesincluding a central processing unit (“CPU”), a random access memory(“RAM”), a display, a read-write mass storage device, a manual inputdevice, and a digital-audio generating integrated circuit (“IC”); andsaid CD-ROM subsystem including: a CD-ROM drive; an audio outputamplifier that is coupled to the CD-ROM drive for receiving an analogaudio signal from the CD-ROM drive; a plurality of CD-ROM controlbuttons for controlling operation of the CD-ROM drive during replay ofaudio compact disks (“CDs”); and an audio-interface IC that is coupledto the digital computer bus of said computer subsystem, to the CD-ROMdrive, and to the CD-ROM control buttons; the audio-interface IC whenthe CD-ROM subsystem is energized: in a first operating mode in whichthe computer subsystem is energized and operating, relaying commands anddata between the digital computer bus of the computer subsystem and theCD-ROM drive; and in a second operating mode in which the computersubsystem is not energized and is inoperative, autonomously respondingto signals received from the CD-ROM control buttons and transmittingcommands to the CD-ROM drive, the commands causing the CD-ROM drive toplay an audio CD present in the CD-ROM drive.
 2. The digital computer ofclaim 1 wherein said CD-ROM subsystem further comprises an audio switch,which operates in response to a control signal received from theaudio-interface IC, for coupling the analog audio signal of the CD-ROMdrive to the digital-audio generating IC included in the computersubsystem when the audio-interface IC operates in the first operatingmode, and for isolating the analog audio signal of the CD-ROM drive fromthe digital-audio generating IC included in the computer subsystem whenthe audio-interface IC operates in the second operating mode.
 3. Thedigital computer of claim 1 wherein said CD-ROM subsystem furthercomprises an icon display that operates in response to a control signalreceived from the audio-interface IC for indicating operating status ofthe CD-ROM drive.
 4. The digital computer of claim 1 wherein said CD-ROMsubsystem further comprises a track-number display that operates inresponse to control signals received from the audio-interface IC forindicating playing status of an audio CD present in the CD-ROM drive. 5.The digital computer of claim 1 wherein during operation of theaudio-interface IC in the second operating mode, when the CD-ROM drivehas been idle for a first pre-established interval of time, theaudio-interface IC enters a lower power operating mode, and transmits acommand to the CD-ROM drive that places the CD-ROM drive in a lowerpower operating mode.
 6. The digital computer of claim 5 wherein duringoperation of the audio-interface IC in the second operating mode, whenthe CD-ROM drive has been idle for a second pre-established interval oftime, the audio-interface IC turns off electrical power to the CD-ROMdrive.
 7. The digital computer of claim 6 wherein the audio-interface ICturns off electrical power to the audio output amplifier upon turningoff electrical power to the CD-ROM drive.
 8. The digital computer ofclaim 1 wherein the audio-interface IC supplies a volume control signalto the audio output amplifier.
 9. The digital computer of claim 1wherein the computer subsystem further comprises a System Management Bus(“SMBus”), the audio-interface IC exchanging commands and data with thecomputer subsystem via the SMBus.
 10. The digital computer of claim 1wherein the audio-interface IC has a third operating mode in which thecomputer subsystem is energized and operating and the audio-interfaceIC, as permitted by a computer program executed by the CPU included insaid computer subsystem, responds to signals received from the CD-ROMcontrol buttons and transmits commands to the CD-ROM drive, the commandscausing the CD-ROM drive to play an audio CD present in the CD-ROMdrive.
 11. The digital computer of claim 1 wherein said audio-interfaceIC includes a multiplexer that when enabled couples the CD-ROM drive tothe digital computer bus of the computer subsystem, and that whendisabled isolates the CD-ROM drive from the digital computer bus of thecomputer subsystem, whereby when the multiplexer isolates the CD-ROMdrive from the digital computer bus of the computer subsystem the CD-ROMdrive may be removed from the digital computer to be replaced thereinwith another device.
 12. An audio-interface IC adapted for use in adigital computer having both a computer subsystem and a CD-ROMsubsystem: the computer subsystem including a digital computer bus viawhich various digital computer devices that may be included in thecomputer subsystem exchange commands and data, such devices including aCPU, a RAM, a display, a read-write mass storage device, and a manualinput device; and the CD-ROM subsystem including: a CD-ROM drive; anaudio output amplifier that is coupled to the CD-ROM drive for receivingan analog audio signal from the CD-ROM drive; and a plurality of CD-ROMcontrol buttons for controlling operation of the CD-ROM drive duringreplay of audio compact disks (“CDs”); the audio-interface ICcomprising: a host-bus interface adapted for coupling theaudio-interface IC to the digital computer bus of the computersubsystem; a CD-ROM interface adapted for coupling the audio-interfaceIC to the CD-ROM drive; a control-button logic adapted for coupling theaudio-interface IC to the CD-ROM control buttons and for receivingelectrical signals from the CD-ROM control buttons; the audio-interfaceIC when the CD-ROM subsystem is energized: in a first operating mode inwhich the computer subsystem is energized and operating, relayingcommands and data between the digital computer bus of the computersubsystem and the CD-ROM drive; and in a second operating mode in whichthe computer subsystem is not energized and is inoperative, autonomouslyresponding to signals received from the CD-ROM control buttons andtransmitting commands to the CD-ROM drive, the commands causing theCD-ROM drive to play an audio CD present in the CD-ROM drive.
 13. Theaudio-interface IC of claim 12 wherein the CD-ROM subsystem furtherincludes an audio switch, the audio-interface IC being adapted forsupplying a control signal to the audio switch for coupling the analogaudio signal of the CD-ROM drive to a digital-audio generating ICincluded in the computer subsystem when the audio-interface IC operatesin the first operating mode, and for isolating the analog audio signalof the CD-ROM drive from the digital-audio generating IC included in thecomputer subsystem when the audio-interface IC operates in the secondoperating mode.
 14. The audio-interface IC of claim 12 wherein theCD-ROM subsystem further includes an icon display, the audio-interfaceIC being adapted for supplying a control signal to the icon displaywhich effects an indication of an operating status of the CD-ROM drive.15. The audio-interface IC of claim 12 wherein the CD-ROM subsystemfurther includes a track-number display, the audio-interface IC beingadapted for supplying control signals to the track-number display forindicating playing status of an audio CD present in the CD-ROM drive.16. The audio-interface IC of claim 12 wherein during operation of theaudio-interface IC in the second operating mode, when the CD-ROM drivehas been idle for a first pre-established interval of time, theaudio-interface IC enters a lower power operating mode, and transmits acommand to the CD-ROM drive that places the CD-ROM drive in a lowerpower operating mode.
 17. The audio-interface IC of claim 16 whereinduring operation of the audio-interface IC in the second operating mode,when the CD-ROM drive has been idle for a second pre-establishedinterval of time, the audio-interface IC turns of f electrical power tothe CD-ROM drive.
 18. The audio-interface IC of claim 17 wherein theaudio-interface IC turns off electrical power to the audio outputamplifier upon turning off electrical power to the CD-ROM drive.
 19. Theaudio-interface IC of claim 12 wherein the audio-interface IC supplies avolume control signal to the audio output amplifier.
 20. Theaudio-interface IC of claim 12 wherein the computer subsystem furtherincludes a System Management Bus (“SMBus”), the audio-interface IC beingadapted for exchanging commands and data with the computer subsystem viathe SMBus.
 21. The audio-interface IC of claim 12 wherein theaudio-interface IC has a third operating mode in which the computersubsystem is energized and operating and the audio-interface IC, aspermitted by a computer program executed by the CPU included in thecomputer subsystem, responds to signals received from the CD-ROM controlbuttons and transmits commands to the CD-ROM drive, the commands causingthe CD-ROM drive to play an audio CD present in the CD-ROM drive. 22.The audio-interface IC of claim 12 wherein the audio-interface ICincludes a multiplexer that when enabled couples the CD-ROM drive to thedigital computer bus of the computer subsystem, and that when disabledisolates the CD-ROM drive from the digital computer bus of the computersubsystem, whereby when the multiplexer isolates the CD-ROM drive fromthe digital computer bus of the computer subsystem the CD-ROM drive maybe removed from the digital computer to be replaced therein with anotherdevice.